I've got a 97 LT1 A4 AUTO and the battery gauge sometimes goes into the red. It's a new battery with clean terminals and wires. I check on the fuse links on the left and get 12.9 to 13.5 volts. and when the gauge dips down low i can turn on my headlights and it'll bring it up. Did I lose a diode in the alternator? what could this be?

 

I had the same problem but then i have a March u.d pulley.I then ordered a o.d alt one and it did bring up the volts and never goes into the red area.Also chk your belt for tightness,etc as this can affect things.

 

At normal operating idle, the voltage at the battery should be around 13.8~14.2 volts... If not then the alternator might be going out...